1. Field of the Invention
The present invention relates to a method of detecting the floating amount of an original for an image reading apparatus, a method of image processing using the same, and an image reading apparatus, in particular to a method of detecting the floating amount of an original placed on an original table of an image reading apparatus having a CIS optical system from read image information and an image reading apparatus using the method.
2. Description of the Related Art
Two types of optical systems are mainly known as optical systems for use in image reading apparatuses. One of them is a “reduction optical system” in which a one-dimensional photoelectric transducer having a width smaller than the reading width and a reduction imaging lens are used in combination, and the other is a “contact 1× magnification optical system (commonly called a contact image sensor, which will be hereinafter referred to as CIS optical system)” in which a one-dimensional photoelectric transducer having a width substantially the same as the reading width and an erection 1× magnification lens array are used in combination.
FIG. 11 illustrates the basic construction of the reduction optical system. The reduction optical system includes an original 102 placed on an original glass plate 101 and illuminated by a light source not shown in the drawing and a reduction imaging lens 104 that forms an image of the original 102 on a one-dimensional photoelectric transducer 103 such as a CCD. FIG. 11 also illustrates the optical path length L1 of this optical system.
FIG. 12 illustrates the basic construction of the CIS optical system. The CIS optical system includes an erection 1× magnification lens array 114 that forms an image of an original 112 placed on an original glass plate 111 and illuminated by a light source not shown in the drawing on one-dimensional photoelectric transducer 113. FIG. 12 also illustrates the optical path length L2 of this optical system.
The optical path length of the reduction optical system tends to be longer than that of the CIS optical system because of constraints on the angle of field of the reduction imaging lens. Floating of the original 102, 112 from the original glass plate 101, 111 indicated by arrows in FIGS. 11 and 12 leads to a change in the imaging condition on the light receiving element. The change in the imaging condition is smaller in the reduction optical system than in the CIS optical system, because the reduction optical system has a large depth of focus.
On the other hand, in the CIS optical system, since constraints on the angle of field of the erection lx magnification lens array 104 is small, the optical path length can be made very short, and the overall thickness and size of the unit can be made small. On the other hand, the depth of focus of the CIS optical system is small.
FIG. 13 shows comparison of the depth of focus between the reduction optical system and the CIS optical system. Since the reduction optical system has a large depth of focus, MTF is characteristically not so sensitive to the floating amount of the original as shown by the thick solid curve in FIG. 13. On the other hand, since the CIS optical system has a small depth of focus, MTF is sensitive to the floating amount of the original and liable to decrease as shown by the broken curve in FIG. 13.
Originals read by the image reading apparatus are commonly flat paper sheets, and situations in which an original is floating when image reading is performed frequently occur in common usage of the image reading apparatus. A case where floating of an original is occurring is illustrated in FIG. 14.
The original 142 placed on the original glass plate 141 is illuminated by an illumination device 145 accommodated in a carriage 146. Light reflected by the original 142 is read by a line sensor through a CIS optical system 144. The carriage 146 is driven by a driving device (not shown) to shift in the direction indicated by an arrow (sub-scanning direction) to effect two-dimensional scanning, whereby an image information of the original is obtained. The image information thus obtained is transmitted to and processed by an image processing apparatus, which is not shown in the drawings.
The original 142 is placed in such a way as to abut an original abutting portion 147. However, the original 142 may sometimes override the original abutting portion as can be often the case if the size of the original 142 is larger than the original glass plate 141. In cases where a paper clip 148 or a binding clip 149 is attached to the original, floating of the original by an amount equal to or half the thickness of such a clip will occur. Furthermore, in cases where the original is a plurality of bundled sheets or where the original has a wrinkle resulting from a crease made in the past, floating of the original will occur.
Defocus resulting from floating of an original leads to an undesirable result of reading by the image reading apparatus. In cases where a read image including letter (or character) information is subjected to optical character recognition processing (Optical Character Reader, which will be hereinafter referred to as OCR processing), such defocus leads to, for example, an increase in the probability of erroneous reading. This is a considerable problem to be solved as a matter of information processing.
As a conventional image reading apparatus using a CIS optical system, for example, an apparatus disclosed in Japanese Patent Application Laid Open No. H11-215329 is known. This apparatus is configured to estimate the floating amount of original from a read output value in a predetermined white background area during scanning of the surface of an original and to adjust an image correction coefficient.
As an image reading apparatus using neither a reduction optical system nor CIS optical system, an image reading apparatus disclosed in Japanese Patent Application Laid-Open No. 2009-246623 is known. This apparatus is intended to solve the two problems. This apparatus has two rows of optical systems arranged in a staggered manner in the main scanning direction. The relative reading optical axes of the two rows are incline in the sub-scanning direction to detect the floating amount of an original.
In the prior art disclosed in Japanese Patent Application Laid Open No. H11-215329, the reading output value in the white background changes depending on the manufacturing variation and/or stability of the illumination device. Furthermore, smudges in the white background can lead to erroneous processing. This prior art method cannot be applied, by its nature, to images having no white background, and therefore cannot be used when reading an image printed over the entire area of the sheet. The prior art disclosed in Japanese Patent Application Laid-Open No. 2009-246623 requires, as will be easily supposed, a considerably increased number of constituent parts in the optical system and needs longer time for adjustment and other operations than before due to the complexity of the apparatus.